virnetdevbandwidth: Compute quantum value

virnetdevbandwidth: Compute quantum value

I've noticed couple of warning in dmesg while debugging
something:

[ 9683.973754] HTB: quantum of class 10001 is big. Consider r2q change.
[ 9683.976460] HTB: quantum of class 10002 is big. Consider r2q change.

I've read the HTB documentation and linux kernel code to find out
what's wrong. Basically we need to pass another argument
"quantum" to our tc cmd line because the default computed by HTB
does not always work in which case the warning message is printed
out.

You can read more details here:

http://luxik.cdi.cz/~devik/qos/htb/manual/userg.htm#sharing

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>